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2 Overview –Types of cloud storage –Building cloud-scale storages –Challenges: theoretical considerations –Dealing with the challenges Based on Moving to the Cloud by Dinkar Sitaram & Geetha Manjunath, to be published by Elsevier

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17 Relaxed consistency –Consistency can be relaxed Weak consistency: system does not guarantee to return consistent results Eventual consistency: if no further updates, system will become consistent. If updates are infrequent, can wait for some time to get consistent value Read your writes consistency: a client performing a read after a write will always see its own updates Session consistency: consistency within a session –Amazon S3 US Standard Region: Eventual consistency US West, EU, Asia Pacific Regions: Read your writes consistency for new object creation, eventual consistency for writes and deletes –Reference: Eventual Consistency by Werner Vogel, Communications of the ACM, January 2009